1. Field of Invention
The present invention relates to a metal etching process. More particularly, the present invention relates to a method for improving the differential etching rate of metallic vias caused by geometric effects.
2. Description of Related Art
In semiconductor manufacture, there are two main methods of forming metallic conducting lines above a semiconductor substrate. One method of forming metallic conducting lines is to etch away portions of the dielectric layer above a substrate to form a trench, then depositing metal into the trench followed by removing the unwanted metal above the dielectric layer. Alternatively, a metallic layer is formed over the substrate first, then the metallic layer is etched to form conductive lines, and finally back filling a layer of dielectric material over the conductive lines. Plasma etching is now one of the most commonly used etching methods for forming, metallic lines. Principally, plasma etching is a method that utilizes plasma to break up the reactive gas molecules into reactive ions. These reactive ions then impinge upon films that are exposed to the plasma, and thereupon the ions will react chemically with the film material. The compound generated by ion bombardment and chemical reaction is quite volatile, and can be easily sucked away by the vacuum pump in a vacuum system, thereby layer upon layer of thin films are removed.
Microloading is always a major problem that affects the etching rate of a metal etcher that uses high-density plasma. When vias of different widths are simultaneously etched using the same etcher, problems will generally arise. This is because it is much easier to etch a wide-open via than a narrow-dense via. In other words, a wide-open via has a higher etching rate than a narrow-dense via. This is referred to as the microloading effect. Due to a difference in etching rate, it often happens that when a wide-open via has etched to the required depth, the narrow-dense via has not quite reached the same target depth. On the other hand, if a longer period is given to etch the narrow-dense via until the required depth is reached, the wide-open via would have already etched past the required depth causing a condition of over-etch.
FIG. 1 is a cross-sectional view of an ideal metal etch structure. As shown in FIG. 1, a metallic layer 11 and a photoresist layer 12 are formed over a substrate 10, then a pattern is formed in the photoresist layer 12 to expose the desired etching locations. Some etching locations are wide for etching a wide via 13a, while others are narrow for etching a narrow via 14a. Yet, the desired etching depth h of all those vias is the same. However, when the actual etching is performed, different vias will have different etching rates. Etching will be much easier for a wide via 13a than a narrow via like 14a. Hence, the etching rate for the wide via 13a will be much higher than the narrow via 14a. The result of etching is referred to in FIGS. 2A and 2B. FIG. 2A is a cross-sectional view showing the structure after the wide via has attained the required etching depth h. Because the etching rate of the narrow via 14a is smaller than the wide via 13a, the narrow via 14a has only etched to a depth of h1 when the wide via 13a has reached a depth h, where h is greater than h1. On the other hand, FIG. 2B is a cross-sectional view showing the structure after the narrow via has attained the required etching depth h. Because the etching rate of the narrow via 14a is smaller than the wide via 13a, a much longer etching time is required to reach the depth h. Therefore, by the time the narrow via 14a has etched to the depth h, the wide via 13a would have over-etched to a depth h2, where h2 is greater than h.
Such differences in the etching rate of metal not only affects the production of vias, but can also lead to a tapering cross-section as shown in FIG. 3 due to insufficient selectivity between metal and photoresist. FIG. 3 is a cross-sectional view showing the structure obtained by etching whose etching selectivity ratio of metal/photoresist is low. When the etching selectivity ratio of metal/photoresist is low, as the metal is being etched in an etching operation, the photoresist layer will also be simultaneously etched, and the result are vias 13b and 14b having a tapering cross-section.
In light of the foregoing, there is a need in the art to provide a better metal etching process.